Photoshop will run v.fast on a G4800 provided you have lots and lots of RAM. I'm still on the standard 256mb that came with my G4800imac and I'm finding that performance is definitely restricted because of that.

If you can afford it go for the 512 RAM, but WOW are they expensive at the moment.

What made you go over to Apple anyway? Seduced by the best looking computer ever